1. Field of Invention
The invention relates to digital imaging using time-to-threshold A/D conversion, particularly to elimination of measurement errors due to different cells having mismatched components and being exposed to differing amounts of incident energy.
2. Description of Prior Art
Time-to-threshold A/D conversion in digital imaging is well-known in the prior art. The approach is most useful in very large (by sensor count) imaging arrays with analog, mixed-signal, and digital logic circuits incorporated onto a single chip. The dominant technology for such systems is CMOS, a popular fabrication process that is widely used to make digital chips such as microprocessors and memories.
Previously, A/D conversion in digital imaging was substantially independent of the imaging process. Sensors in an array would be exposed to incident energy simultaneously, for a given common exposure time. Then, each sensor output signal would be passed out of the array to a separate general-purpose A/D converter.
However, in time-to-threshold A/D conversion, elapsed time is tracked during exposure. When a sensor output signal reaches a threshold level, the elapsed time since the start of exposure is taken as the digital representation of the analog sensor output response.
Sports analogies are useful in understanding the difference. Prior art array-external A/D conversion with general purpose A/D converters is similar to a fixed-time race such as the “24 Hours of Le Mans”. The digital measurement is of how far the sensor output signal goes in a fixed amount of time.
On the other hand, time-to-threshold A/D conversion is similar to a fixed-distance race such as a 100 meter sprint. The digital measurement is of how much time is required to go from start to finish.
Several U.S. patents describe various types of time-to-threshold A/D conversion for digital imaging, including U.S. Pat. No. 5,650,643 issued to K. Konuma, U.S. Pat. No. 6,587,145 issued to A. Hou, and U.S. Pat. No. 6,559,788 issued to C. Murphy. U.S. Pat. No. 5,461,425 issued to B. Fowler and A. El Gamal describes an early proposal for putting A/D converters in an imaging array as a way of avoiding having to pass analog signals to array-external A/D converters.
The advantages and disadvantages of some of these patents are described in U.S. Pat. No. 6,680,498 issued to R. Guidash. U.S. Pat. No. 6,680,498 also discusses several prior art methods that use multiple images or variants of standard imaging techniques to enhance the performance of digital imaging systems, notably the work of O. Yadid-Pecht and his colleagues presented at the 1997 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors.
Notwithstanding, none of the prior art on time-to-threshold A/D conversion in digital imaging takes into account the undesirable effects known as fixed-pattern noise (FPN).
FPN typically refers to static or slowly-changing variations in the behavior of different sensor cells in an array. In most systems, these variations are largely independent of differences in incident energy at the sensors. Rather, they depend on component mismatch. Component parameter mismatch can result from manufacturing or age effects. In CMOS, many circuit parameters depend on size ratios—for instance, transistor gains are functions of channel width-to-length ratios—so that size errors lead to component mismatch.
FPN is a particular problem in high-precision imaging systems, as the “noise” pattern may be much stronger than the weak incident energy variations which such systems aim to detect.
In prior art CMOS image sensor arrays with a pre-determined common exposure time followed by A/D conversion, FPN can be eliminated using so-called “double-sampling” (DS) or “correlated double sampling” (CDS).
With DS, a reference “dark” measurement is taken, as well as a post-exposure “light” measurement. The “light” measurement includes the effects of incident energy during exposure, whereas the “dark” measurement does not. With CDS, a first measurement of a sensor output is taken after sensor initialization but before exposure, and a second measurement is taken after exposure.
In both DS and CDS, computing a difference between two measurements for the same sensor allows cancellation of any common terms. For CDS, fixed-pattern errors and initialization errors are substantially cancelled, whereas in DS only fixed-pattern errors are corrected. Initialization errors can occur when the pre-initialization state of a sensor affects the actual state reached during the finite initialization time.
DS and CDS can be implemented digitally after A/D conversion of measured sensor outputs, or via storage of an analog first measurement followed by analog subtraction of a second measurement prior to A/D conversion.
Both DS and CDS effectively implement subtraction of a noise-only measurement from a noise-plus-signal measurement in order to obtain a signal-only measurement. Insofar as “noise” is repeatable, such a differential measurement technique is a simple yet elegant solution to the problem of signal extraction.
Imaging systems with time-to-threshold A/D conversion may suffer from both FPN and input-dependent (i.e. incident energy dependent) errors. Slow comparing circuits may provide a suitable digital indicator signal with some delay. The amount of delay may depend on the strength of the comparing circuit input. A strong input may quickly exceed the threshold level and so over-drive the comparing circuit, resulting in a short delay. A weak input signal may hover near the threshold level and so not over-drive the comparing circuit as much, resulting in a longer delay.
Time-to-threshold A/D conversion in digital imaging offers the possibility of high-precision imaging at low cost, but the prior art has so far neglected removal of FPN and input-dependent delay errors.